The present invention relates generally to a voltage charge pump and more particularly to a voltage charge pump for use in integrated circuits, among others.
Integrated circuits, such as a dynamic random access memory device (DRAM), typically employ one or more voltage pumps which are used to create voltages that are more positive or more negative than the available supply voltages. A DRAM may use two types of voltage pumps. The first type is a Vccp pump which generates a positive voltage that is used by the DRAM, for example, as a boosted wordline voltage. The second type is a Vbb pump (or back-bias voltage pump) which generates a negative voltage that is used, for example, to negatively bias the DRAM's substrate.
Charge pumps typically use voltages that exceed |Vcc| and thus require specialized transistors. For example, Vbb pumps typically use high voltages (e.g., greater than |Vcc|) to pump the substrate to a negative voltage. Thus, Vbb pumps require the use of specialized transistors having a thick-gate oxide (e.g., approximately 50-60 Å) and/or a unique doping profile. The thick-gate oxide and unique doping profile help to prevent breakdown and punch-through (among others) when the transistors are subjected to the high voltages. These specialized transistors may be referred to as “thick-gate oxide transistors” and/or “thick-gate transistors”. FIG. 16 illustrates a prior art Vbb pump utilizing thick-gate transistors. The thick-gate transistors are required because, in certain instances, the absolute value of the drain-to-source voltage (|VDS|) may exceed VCC (i.e., which causes the breakdown and punch-through as discussed above).
Thick-gate transistors, however, have certain performance shortcomings as compared to thin-gate transistors (the use herein of the terms “thin-gate oxide transistor” and/or “thin-gate transistors” refers to common transistors, for example, transistors having an gate oxide thickness of approximately 25-30 Å and/or having a common doping profile). For example, thick-gate transistors typically pass less current than a thin-gate transistor having a similarly-sized channel width. Thus to charge a capacitor in an equivalent amount of time as the thin-gate transistor, the channel width of the thick-gate transistor must be increased to allow more current to flow. Additionally, the threshold voltage (i.e., Vt) of a thick-gate transistor is higher than that of the thin-gate transistor. Thus, thick-gate transistors require more die space and consume more power during normal operation than an equivalent performing thin-gate transistor.
Several attempts have been made to use thin-gate transistors for pump circuits. FIGS. 17 and 18 illustrate a single stage prior art Vccp pump and a single stage prior art Vbb pump, respectively. Referring to the VCCP pump in FIG. 17, signals PH1 and PH2 are non-overlapping active-low clock signals which are operated such that the gate-to-source voltage (VGS), the gate-to-drain (VGD), and the drain-to-source voltage (VDS) of transistors M2 and M4 do not exceed VCC. Additionally, signals PH1 and PH2 also prevent the absolute value of the gate-to-source voltage (|VGS|) and the absolute value of the gate-to-drain (|VGD|) of transistors M1, M3, M5, and M6 from exceeding VCC, and further prevent the absolute value of the drain-to-source voltage (|VDS|) of transistors M1 and M3 from exceeding VCC. Thus, thin-gate transistors may be used in the single stage Vccp pump illustrated in FIG. 17. However, when PH1 (PH2) goes low, PMPN (PMPN2) may briefly transition below VCC thereby causing charge injection within transistors M2 and M4.
Referring to the single stage Vbb pump in FIG. 18, signals PH1 and PH2 are non-overlapping active-high clock signals which are operated such that the gate-to-source voltage (VGS), the gate-to-drain (VGD), and the drain-to-source voltage (VDS) of transistors M2 and M4 do not exceed VCC. Additionally, signals PH1 and PH2 also prevent the absolute value of the gate-to-source voltage (|VGS|) and the absolute value of the gate-to-drain (|VGD|) of transistors M1, M3, M5, and M6 from exceeding VCC, and further prevent the absolute value of the drain-to-source voltage (|VDS|) of transistors M1 and M3 from exceeding VCC. Thus, thin-gate transistors may be used in the single stage Vbb pump illustrated in FIG. 18. However, when PH1 (PH2) goes high, PMPN (PMPN2) may briefly transition above VSS thereby causing charge injection within transistors M1, M3, M5, and M6.
Accordingly, a need exists for a voltage pump that utilizes thin-gate transistors, increases the pumping capacity/efficiency of the voltage pump, and overcomes the limitations inherent in prior art.